1. Field of the Invention
The present invention relates to a method of manufacturing free standing shapes of refractory metals and their silicides through a Chemical Vapor Deposition process, and particularly to a method of forming high quality refractory parts with high corrosion resistance.
2. Description of the prior art
Refractory metal parts are usually made through powder metallurgy processes. Die pressing and sintering of powders is a well established process route. More recently, injection moulding has extended the processing technology of metal powders to the production of more complex, near net shape components at density levels much higher than before. (For more details about this technology, it can be referred to the article of Robert W. MESSLER entitled "Powder injection moulding, research at Rensselear": an update Metal Powder Report, vol. 44, May 5, 1989-Pages 362-368, or to the article of P.J. JAMES entitled "Injection molding of metal powder", Metal Powder Report, vol. 44, May 5, 1989, pages 369-372).
However, although metal powder injection moulding is a potential and promising method, several problems and drawbacks remain. Such injection molding process requires large size sophisticated equipment, high investment cost and high production rates. The process itself is complicated (the debinding step requires a careful heating in a controlled atmosphere to optimize said step which makes it critical to achieve high quality products), time consuming (up to a few days for the overall process), energy consuming (sintering temperature is at least 1500.degree. C. for refractory metals such as W, Mo or Ta). Finally, it is a poor yield process wherein shrinkage occurs during the sintering steps making it difficult to obtain tight dimensional tolerance without a post-machining step of workpiece.
In order to avoid those drawbacks, it has been recently suggested to use a chemical vapor deposition (CVD) process for small shaped and/or thin parts : it is suggested in "Advanced P.M. Materials at Fulmer Research" Metal Powder Report, Vol 44, Sep. 9, 1989 pages 607-611 to carry out Tungsten deposition on a support by reduction of Tungsten fluoride with hydrogen H.sub.2 under CVD conditions. The same has also been suggested in Japanese patent application JP 63/286574 of SASAKI et al.
However, the use of refractory metal containing parts, either obtained by powder metallurgy or CVD routes, is limited under high temperature oxidizing environments under which oxidation of parts becomes then a real problem. In order to limit such phenomena, it is known to deposit and additional corrosion resistive layer typically refractory metal silicide such as MoSi2, well-known for its anti-corrosion properties. Such additional layers are deposited by usual techniques such as pack cementation, slip pack or slurry methods. In this case, refractory metal parts withstanding to corrosive atmosphere are manufactured through a multi-step process.